Currently, the standardization of the W-CDMA (Wideband Code Division Multiple Access), as a format of a third generation mobile communication system, is progressing in the 3GPP (Third Generation Partnership Project), which is an international standard. As one of themes of this standardization, an HSDPA (High Speed Downlink Packet Access) which provides high speed transmission at scores of Mbps in the downlink has been prescribed. The HSDPA uses an adaptive modulation and coding system AMC (Adaptive Modulation and Coding). The AMC is featured by adaptive switching between the QPSK (Quadrature Phase Shift Keying) system, which is 4-valued quadrature phase shift modulation, and the QAM (Quadrature Amplitude Modulation), which is a 16-valued quadrature amplitude modulation, depending upon radio environments prevailing between the base and mobile stations.
The HSDPA adopts an H-ARQ (Hybrid Automatic Repeat reQuest) system. In the H-ARQ system, if a mobile station has detected an error in data received from a base station, data is re-sent from the base station on request from the mobile station. The mobile station performs error correction and decoding using both the received data and the re-sent data. Thus, in the H-ARQ, the mobile station effectively utilizes the data, already received, even if it is corrupted with errors, thus improving the gain of the error correction and decoding to suppress the number of times of data re-send operations from increasing.
If, in the W-CDMA communication system, pursuant to 3GPP, the HSDPA is used, the AMC scheme is adopted. The AMC is such a technique that adaptively changes the code rate in error correction or the modulation scheme in response to variations in the network quality. In the AMC technique, the encoding/modulation scheme which provides for a higher rate is used in case of a satisfactory network quality to elevate the transmission rate. If the network quality is not satisfactory, the encoding/modulation scheme, providing for a low rate, is used to lower the transmission rate to assure reliable packet transmission.
In general, the receiving scheme, adapted to cope with HSDPA, is of two configurations. One is the equalization (Equalization) scheme which moderates the adverse effect of multiple paths, and the other is the RAKE receiving scheme which maximizes the ratio between the combined multipath signals and the noise without taking account of the interference between multiple signal paths. The equalization receiving scheme is in need of voluminous processing operations, such as those by inverse matrix calculations, thus increasing the power consumption of the entire apparatus. On the other hand, the RAKE receiving scheme is simple to implement and low in power consumption. Heretofore, in order for the receiving side to get an optimum receiving quality, the above two receiving schemes are combined together and switched depending upon prevailing communication environments by way of exercising optimization control (in Patent Documents 1 and 2, for example).
To improve the throughput of the entire system, Patent Document 1 discloses selecting the RAKE receiving scheme or the linear equalization receiving scheme based upon the signal to interference ratio, referred to below as ‘SIR’, Doppler shift or the interference power. Patent Document 2 discloses providing a copy portion (Cyclic Prefix, abbreviated to CP) of an information signal in a transmission slot during the high-speed transmission, such as to assure processing for effective equalization in a receiving unit. During the low-speed transmission, data is transmitted without providing the CP in the transmission slot. As a result, a satisfactory communication quality may be obtained under a variety of communication environments. These conventional techniques switch between the receiving schemes to improve the communication quality.